At this point, probably over 50%. It won't happen this time, but you could buy a ton of B.SELL cheap in case it does happen again. If it does it will be multiple changes down the road.
My very rough step-by-step calculation... {for each unit:} BDD should hold 200 days of dividends. Current dividend is 0.00006187 BTC. BDD should hold 0.012374 BTC. BDD currently holds 0.00784292 BTC.
B.SELL will pay dividend if B.MINE dividend becomes 0.00784292/200=0.00003921 BTC. This will happen for difficulty 64.13 giga. That means 64.13/40.64 = 1.578 ... We need 57.8% increase.
The number of BDD units will obviously change, but this should be rough estimation. Please correct me if I made some serious mistake.